(a) Field of the Invention
The present invention relates to thin supported electrodes of lithium and to a process for preparing these electrodes. More specifically, the present invention is concerned with a process for manufacturing thin electrodes of lithium, lithium alloys or doped lithium, which are supported on a sheet of an electronically conductive material, as well as with the electrodes obtained by this process.
(b) Description of Prior Art
Since the advent on the market of rechargeable lithium generators (Moli Energy Ltd., Burnaby, B.C. Canada) and the recent emergence of polymer electrolyte, all solid batteries based on the development of lithium generators have evolved very rapidly in the last few years. These new systems all rely on a technology which is based on thin films where current densities are low with the result that they promote a good redepositing and cycling of the lithium electrodes. This tendency has increased the need to produce thinner and thinner lithium electrodes: .about.100.mu. for liquid-electrolyte batteries and &lt;30.mu.=&gt;.about.1.mu. in the case of polymer electrolytes.
The utilization and handling of thin lithium films are relatively easy when the thickness remains about 100.mu.. Commercially produced films are available at a price of the order of US $100 per pound. However, the cost of thinner films increases rapidly since they should then be produced by extrusion followed by lamination; the latter operation is slower and more difficult (high labor cost) with the result that the cost of the lithium produced triples at least. Considering that lithium constitutes a non-negligible portion of the price of the battery, its cost could represent up to 50% of that price. In addition to the higher cost of very thin films (50.mu.), the film also becomes difficult to handle because of the high deformability of lithium resulting from its malleability and its adhesiveness to most usual materials. This means that thin lithium films are extremely difficult to handle in continuous processes of assembling batteries made of superposed films: electrode (+)/electrolyte/lithium electrode.
The technology involved in manufacturing polymer-electrolyte lithium batteries of specific concern in the present invention, is particularly demanding in this respect, since the required thicknesses of lithium, with respect to the characteristics of currently known electrolytes, vary between 30 and about 1.mu.. Means of overcoming this difficulty are known, such as the utilization of double-sided negative electrodes, which allows to use double the required thickness. (Third International Meeting on Lithium Batteries, May 27 to 30, 1986, Kyoto, Japan, Abstract #ST-11). However, if the intention is to produce bipolar type batteries corresponding to the sequence:
Li/Electrolyte/(+)/Ni/Li/Electrolyte/(+)/Ni/Li. . . (+)/Ni,
where nickel is chosen as an example, it becomes necessary to rely on very thin films if an excess of lithium is to be prevented. Excess lithium is in fact detrimental to the cost of the raw material and to the density of stored energy, especially in terms of energy per unit volume; this excess becomes even crucial in the case of batteries designed for room temperature operation, where the quantities of lithium required (1-2 C/cm.sup.2) are very low and correspond to thicknesses varying between 1 and 5.mu..
Various processes have been suggested to produce ultra-thin films of lithium, such as when coating a metal collector. This is the case, for example, of lithium deposit by thermal evaporation, by sputtering or by electron beam. However, these techniques are relatively slow and costly, because they are carried out under high vacuum and under conditions of strict cleanliness. Thin films less than 1 .mu. thick can thus be obtained.
Other processes exist, such as lamination and deposit by transfer on a metallic support, which have been described in U.S. Pat. Nos. 3,756,789, dated Sept. 4, 1973, Inventor: Alder and 3,721,113, dated March 1973, Inventor: Houseplan, or hot coextrusion with a film of plastic material (European Patent Applications Nos. 0 146 241, Park et al., June 26, 1985 and 0 145 498, Cook et al., June 19, 1985). All these processes have serious drawbacks, especially if an attempt is made to apply them to manufacture rechargeable polymer-electrolyte batteries.
On the other hand, there are methods of plating steel sheets by unrolling the latter in a zinc bath. In this connection, reference is made to the following patents:
Japan No. 57-203758, Nippon Steel PA1 Japan No. 57-203759, Nippon Steel PA1 Japan No. 57-203760, Nippon Steel PA1 GB No. 2,080,340, Nippon Steel PA1 Canada No. 1,145,210, Battelle Memorial Institute
The technology proposed in these patents is obviously not adaptable to the production of a thin layer of lithium on a metallic sheet. Galvanization by means of a roller on one side of a steel sheet according to a Nippon Steel process should also be mentioned (L'Usine Nouvelle, December 1986).
For battery applications, the control of the thickness of the lithium films is much more critical than in the galvanization processes. On the one hand, if the lithium layer is too thin, a portion of the collector could be exposed during discharge, resulting in irreversible or at least serious problems during recharge. It is indeed well known that lithium can be redeposited a large number of times (more than 500 cycles) as long as the lithium is redeposited on itself and not on a metallic collector, for example nickel. On the other hand, control of the thickness is absolutely necessary in order to prevent the formation of extra thicknesses during the manufacture of complete batteries since extra thickness is penalizing in terms of cost and accumulated energy. Finally, control of the thickness is necessary in order to ensure a precise balance of the surface capacitance (C/cm.sup.2) of the electrodes when the batteries are mounted in series; otherwise the capacitances of the individual batteries would progress differently during recycling.
The present invention is intended to overcome the above-mentioned difficulties in the use of lithium electrodes and to produce lithium films of various thickness, for example between 40 and about 0.1.mu., rapidly, economically, and in a particularly reproducible manner from one batch to the other.
The present invention also intends to take advantage of the outstanding wetting properties of molten lithium, lithium alloys or doped lithium when used in association with metals such as nickel and copper.
An object of the present invention is to develop a rapid process for producing rolls of lithium spread on a support, preferably metallic or made of other metallized or heat-resistant materials, by using the high speed wetting properties of thin sheets of lithium, at thickness between about 1 and 20.mu..
Another object of the present invention is to benefit from the speed of the process to reduce the time of contact between molten lithium and the support material and to prevent any chemical or thermal attack by the molten lithium.
Another object of the present invention consists in controlling the device used and the unwinding speed of the preferably metallic sheet so as to allow lithium to undergo thermal treatments. Examples include the control of the rate of solidification (micro-cristallinity of lithium), or of the chemical treatments.
Another object of the present invention is to manufacture supported lithium electrodes intended for polymer-electrolyte batteries using molten lithium applied by methods that allow a strict control of the thickness of the lithium deposit.
Another object of the present invention is to ensure thin and reproducible deposits by controlling the thickness and, therefore, the capacitance of the lithium layer; on the one hand, this will reduce the excess of lithium and, on the other hand, it will ensure good electrochemical operation during the cycling of the lithium electrode and of the battery.